Landslides and Slope Mitigation in California's Great Valley...Wait...What?

 

I used to talk to my students about the geological hazards that we face as inhabitants of California's Great Valley (or Central Valley, for those who don't appreciate its actual greatness). I would go down the list of things to worry about: earthquakes, droughts, wildfires, volcanic eruptions, flooding, and so on. But then I somewhat jokingly described things we didn't have to worry about such as hurricanes (Florida's problem), tsunamis (a problem for coastal cities), tornadoes (Oklahoma's problem), and mass wasting (also known by the generic term 'landsliding').

Unfortunately, over the years I've become aware that some of those unlikely hazards actually can be a factor in living in the valley. We've had a fair number of tornadoes in recent years, including two that came within a few miles of my house (they weren't anything like the monsters of Tornado Alley in the Midwest, but still a bit scary). A powerful tropical storm hit Southern California last summer that came up just short of being a hurricane, and the heavy downpours were statewide. And then there is mass wasting (slope failures and landslides). I know of at least two fatalities caused by mass wasting in the last few years. One was a homeless person who had dug a tunnel into a river embankment that later collapsed, and another was a person who was driving along a freeway in heavy rains when the freeway embankment collapsed as a mudflow and spread across the lanes causing a fatal accident.

The Great Valley is famously flat so mass wasting doesn't seem to be much of a danger to those who live here, since landslides and other slope failures require, well, a slope to happen. But the valley is not quite so flat as people may think. The valley is 400 miles long, and most of it is close to sea level. Much of it is low-lying river floodplains, but other sections sit at slightly higher elevations because of complex history of climate change and glacial ice ages over the last 1,000,000 years. These bluffs and terraces protect my city and others nearby because even the worst floods are contained within the floodplains and do not spill over onto the terrace surfaces where cities like Modesto and Turlock have been built near the Tuolumne River.

During the ice ages glaciers never reached the valley floor, but meltwater from the Sierra Nevada glaciers swelled the rivers to several times their average flow, and they carried tremendous amounts of muddy sediment that spread widely across the valley floor building up alluvial fans. When the glaciers receded, the rivers flowed less, but carried clear water that was more capable of eroding the soft sediments of the alluvial fans, forming channels and floodplains several tens of feet deep. Once these channels developed, floods never covered the terraces again. It's the bluffs that form the boundaries of these terraces that provide the conditions that can result in slope failure.

The heavy rains of 2022-23 led to widespread flooding across many parts of California including some real problems on the Tuolumne River Parkway Trail when I regularly go birdwatching. I wrote about these in January of 2023 in the aftermath of one of the biggest storms. The most serious problem was the access road to our town's water treatment plant. It's on the river floodplain about 60 feet below the river terrace. Slumping had caused major cracks to form in and near the pavement.

Eventually the rains subsided and the soil dried up. The slide seemed to stabilize, but the threat to the roadway remained and would eventually have to be dealt with. That is what was new this week: the cranes and were in place to start the slope mitigation process.

The main problem is that the access road traverses unstable debris and soil that slumped in the 2023 event. They would need to re-engineer the slope by rebuilding it from scratch. Their strategy was complicated by the fact that all the equipment and materials had to traverse the very road they were trying to repair. Truckloads of heavy boulders were going down the road every few minutes. Meanwhile a huge long-reach excavator was digging away at the slope below the road!

After digging away and smoothing off the slope they covered it with felt matting and then started piling many tons of boulders on the slope. The boulders are intended to buttress the slope and hopefully keep it stable during future weather events.

It's a lot of work being done to keep a single paved road open, but it's a pretty important road since it provides the only access to the water-treatment plant for the city of Waterford. And thus we are dealing with slope mitigation in what is supposedly the flattest place in the country!

The Merced River of California: A Place for Wildflowers and Landslides (and Yosemite Valley)

The Merced River is one of California's extraordinary waterways. First and foremost, it is the river that carved Yosemite Valley, which is one of the world's most spectacular valleys. You may be forgiven for thinking that glaciers carved Yosemite Valley, and they certainly had an important role, but it was learned years ago that the heavy-duty work of carving the 3,000-foot-deep gorge was done before the ice ages ever arrived 2 million years ago. The glaciers certainly modified the shape of the valley, but the depth was first achieved by water flowing over stone.

We spent a couple of days in the valley at a time when the crowds are few, the snow is in retreat, but the green of spring is still some weeks away. It was neither winter nor spring, but the waterfalls were full and the birds were becoming more active, and we had a marvelous time. This morning brought a sight we had never seen in dozens of previous visits though: the beginning of a controlled burn. 

The native Americans who first lived and hunted in the valley knew the value of fire. They would regularly set fires to the forest and meadows to clear out undergrowth and maintain the extensive meadows on the valley floor. When the park was established in 1890, fire became the enemy and any hint of a fire was quickly put out. As a consequence the meadows shrank from the original 750 acres to less than 100 today. The forest became choked with young trees, and fuel built up on the forest floor. It was a major disaster-in-waiting. In recent years, the park management has sought to return the valley floor to something resembling the environment that existed prior to European colonization. Some of the activity has involved some controversial logging operations, and the controlled burns can interfere with the experience of visitors, but the eventual changes will be a vast improvement over present conditions.

One of our real motivations for the trip actually involved a different part of the Merced River, the deep canyon downstream of Yosemite Valley. The Merced River canyon downstream of the lowest extent of the glaciers is a different place entirely. The vertical walls of glacially-shaped granite give way to a V-shaped river valley covered by vegetation that often obscures an extensive belt of metamorphic rocks that are hundreds of millions years older than the granite intrusions.

While not as "spectacular" as Yosemite Valley, the canyon is not without scenic wonders. Indeed, for a few weeks in the spring the walls of the canyon can explode into a color palette that puts the gray granite cliffs of Yosemite Valley to shame. Wildflowers like golden poppy, lupine, and fiddleneck and many others can cover the slopes from the river to the canyon rim.

The climax (if it comes; it's been a dry year) lies a few weeks in the future. But here and there, the slopes provided a hint of what is to come.

The redbuds are just getting started with their not-red buds. More of a pink/purple shade if you ask me.

The slopes of the gorge are also an environment adapted to fire. In the Mediterranean climate the winters are cool and wet, and the summers are very hot and dry. Fires have always swept through the gorge, but those of recent years have been particularly intense due to the extended droughts and higher temperatures due to global warming.

Ultimately fires are a necessary part of the environment, but they can seem particularly tragic in the short-term, especially when people are killed or developments destroyed. The wildflowers play an important role in the recovery of the slopes, as they are pioneer species that help rebuild nutrients in the soils that can assist in the return of the shrublands and forest. The seed may lie in the soil for years before conditions are right for germination, and then the barren slopes come alive with color.

The Golden Poppy is a particularly visible part of the wildflower population. It is California's state flower, and it was a great choice because it thrives in many if not most of California's diverse ecosystems. They're common in the Coast Ranges, the Great Valley, the Sierra Nevada foothills, the volcanic landscapes of the Cascades and Modoc Plateau, the mountains of southern California, and in the Mojave Desert (the Antelope Valley Poppy Reserve is spectacular right now, from what I am hearing). They can also grow on California's strange serpentine soils that most plants cannot tolerate.

A final sight were looking for was the Ferguson Slide. The slope at this particular turn in the river was always unstable, but in 2006 the entire slope slide downwards and covered the highway with tens of thousands of cubic yards of boulders. Far more debris remains on the slopes above, and for a time there were fears that a truly massive slide could block the river and form a natural dam with a lake several miles long. A number of "fixes" were considered, and in the meantime "temporary" bridges were built to provide one-way access on the busy highway. Temporary in the sense that they have been there for fifteen years and counting.

In the end they have decided to build an "avalanche shed", but for rocks instead of snow. The cost is in excess of $180 million (which at the time was about the entire budget for Yosemite National Park). Nature doesn't take kindly to attempts at altering the physics of the slope, and some of the first attempts at stabilizing the upper slopes were laughably sloughed off. There was an earlier coating of the fencing material that was destroyed soon after application. They spent several years carting off nearly 150,000 cubic yards of metamorphic rock and have again tried to fence in the slope. It will be interesting to see what happens when the construction of the shed actually begins.

The photo above is the most comprehensive picture I've been able to capture. I made sure to be last in line when the traffic light changed and allowed the line of cars to follow the one-lane road. I lingered a few seconds to get the shot, and hurried along to make sure I didn't run into oncoming traffic on the narrow bridge. 

There are many more stories to tell of the Merced River downstream, where it flows through the Mother Lode, the places where it lies trapped in the waters of Lake McClure and Lake McSwain, the dredge pits at Merced Falls, and the slow journey across the floor of the Great Valley where it ends in the San Joaquin River. Those will come another day!

Out in America's Never Never: Barriers and the Thin Line of Civilization

Photo by Mrs. Geotripper

In math, getting from Point A to Point B is hardly a problem. It's a straight line most of the time, or an arc, determined by some kind of equation. Getting from point A to point B can be a problem in a city, since buildings are often in the way, yet a good map or a vocal GPS unit gives you plenty of routes and choices to avoid traffic and delays.

Out in America's Never Never, the vast desert encompassing the Colorado Plateau and Basin and Range provinces, getting from Point A to Point B becomes a true challenge. Mountains may block the route, as the Forty-Niners found during the California Gold Rush (and earlier settlers like my ancestors, the Donners). There might be flat, open ground between one point and the other, but there might be no water. In a pre-automobile era, that was a serious problem. But out of all these examples the Colorado Plateau has one really impressive barrier: the Colorado River.

Photo by Mrs. Geotripper

What's the problem? It's a fair sized river as such things go, the biggest of the American Southwest, but it is hardly a trickle compared to many rivers back east and in the Pacific Northwest. Those rivers were barriers to travel, but numerous ferries and bridges soon solved access problems a long time ago (and numerous boats used the rivers themselves as a highway). The Colorado, though, is a geologically young river. It is rapidly cutting downward through the sedimentary and metamorphic rocks of the recently uplifted continental crust, and the consequence of this is an unnavigable rapid-filled river ensconced in a deep vertical gorge along its entire pathway through the plateau country. Until the Navajo Bridge (above, left) was constructed in 1928 the only place to cross the river between Moab, Utah and Needles, CA, a river distance of 600 miles, was at Lee's Ferry, about five miles upstream of Navajo Bridge (the completion of which put Lee's Ferry out of business). The ferry was the only spot on the river whose approach was not overly complicated by vertical cliffs.

There are more bridges today, but there is still a 300-plus mile stretch between Navajo Bridge and Lake Mead at Hoover Dam where there are no crossings (except the footbridge in Grand Canyon near Phantom Ranch). Travel through the Colorado Plateau country is still complicated by this fact (and frankly, it should remain so).

The paved highways give us a somewhat false sense of easy access, as they must also surmount or circumvent barriers in the landscape formed by river canyons and vertical cliffs. As long as there is gas in the tank, a working air conditioner, and properly tuned engine, we give little thought to how tenuous our control is over our localized environment. A flat tire on a deserted road on a blazing hot day can be life-threatening. And small geological events can have far-ranging consequences.

The original plan for our trip (conceived a year earlier) was to leave the North Rim of Grand Canyon and scoot down the slopes of the East Kaibab Monocline into the Navajo Section of the plateau country. We would cross the Colorado River at Navajo Bridge, turn at Bitter Water onto Highway 89 to Page, Arizona, where we would have a slow-paced tour of Horseshoe Bend, Glen Canyon Dam, and the incredible Antelope Canyon. From there it would be a fairly quick drive to Navajo National Monument, where we would be camping for the night. It was meant to be the most leisurely day of the trip.

Oh well....

The picture above gives a hint about how geology made our day a constant exhausting race against time. The Vermilion Cliffs are a prominent part of the Grand Staircase, revealing early Mesozoic formations, including the Chinle, Wingate and Kayenta. The Chinle, at the base, is composed of easily eroded shale and siltstone, and is subject to slope failure. In the picture above, looking across House Rock Valley, the lower slopes look tilted because they are part of a huge rotational slide.

If you take a big group through this region, you will need to make the camping reservations five or six months in advance, and even at that I nearly missed getting reservations for Arches National Park and the North Rim of Grand Canyon. As such, once the schedule has been established, it can't be easily changed.

But geology can change. Highway 89 from Bitter Water to Page starts out in spectacular fashion, climbing a slope of Chinle to a big cut through the Echo Cliffs, composed of the Wingate, Kayenta and Navajo formations. The view from the upper highway is a stunning panorama of Marble Canyon, the Vermilion Cliffs, and the East Kaibab Monocline that forms the margin of the Kaibab Plateau and Grand Canyon.

Photo by Mrs. Geotripper

The problem for the road engineers was getting across the Chinle slopes. The Chinle doesn't like highways on the flatlands (swelling and settling of the clays turns the best planned highways into roller-coasters), but it especially doesn't like roads on slopes.

Ultimately they chose to put the road across an ancient landslide not unlike the one in the Vermilion Cliffs above. It is especially visible in Google earth images, where a white outcrop of Navajo Sandstone can be seen well out of place next to the highway (below).

Source: GoogleEarth

In February, a portion of the old slide gave way, destroying a significant stretch of the highway. This wasn't the kind of slide that could be fixed easily, and the road will be out of service for a long time. The Echo Cliffs constitute a significant barrier to east-west travel, and between Bitter Water and Tuba City, only one dirt road crosses it. The state is in the process of paving the track, but it was nowhere near done when we passed through.

Source: Lee Allison, at http://arizonageology.blogspot.com/2013/03/adot-estimates-35-million-to-repair.html

If we were going to see the Navajo Bridge and the Colorado River, we would need to make a 140 mile detour to get to Page and our other objectives. And then backtrack around 70 miles.We barely made it to our tour appointment at Antelope Canyon at 3 PM. We got to see Horseshoe Bend, but the visit to Glen Canyon was cut very very short, and we didn't reach camp until well after dark.

Source: Arizona Department of Transportation

It was a long day...but spectacular, just the same. More later!

For more information about progress with Highway 89, check out http://www.azdot.gov/us89/

Geologic Change Along the Big Sur Coastline: Julia Pfeiffer Burns State Park

Gee, I bet no one has ever thought to take a picture here before...

Okay, maybe a few people have chosen to take pictures of the scene above. The view of McWay Falls in Julia Pfeiffer Burns State Park along the Big Sur Coastline of Central California is iconic. It is pretty much visible from only one trail and overlook, since the beach is (quite properly) closed to visitation (try googling an image from any other angle). But is there a prettier waterfall anywhere?

Actually, though, the proper name for McWay Falls is a tidefall, since during high tide the water falls directly into the sea (only two waterfalls in California do this, and only a few dozen in the world). Taking the longer view, it's not hard to see how a waterfall here could drop into the sea. The Big Sur Coastline is a mountain range that falls directly into the sea. The Santa Lucia Range reaches elevations of more than a mile, and at Julia Pfeiffer Burns State Park, a 4,000 foot high peak is only two miles from the beach as the crow flies (or falls).

The Santa Lucia range and the rocks along the coast are part of the Salinian Block, an assemblage of granitic and metamorphic rocks that is related to the Sierra Nevada. The rocks formed in a magmatic arc several hundred miles to the south. They were carried north during the last 29 million years by movements of the San Andreas and related fault systems. The combination of tough, hard-to-erode rocks and rapid tectonic uplift has turned the Big Sur into one of the most beautiful coastlines in the world.

This is a zoom on McWay Creek cove in the picture above. To get a sense of scale, look for the tourists on the left hand margin of the photo halfway up.

McWay Falls and the cove it is in are not pristine wilderness. The land was in private hands for many years, and there was once a house where the viewpoint is today. The owners planted palms and eucalyptus, as well as numerous other non-native plant species. The park is in the slow process of removing the invasive species. I'm not against the idea. There is a redwood grove just up the canyon (McWay Falls is very close to the southern edge of the natural range of the trees). It's hard to imagine humans improving the natural vegetation here.

There have been geologic changes here too, and in a sense they are "non-native" as well. The construction of Highway 1 through the Big Sur country was completed in 1937, and the steep topography necessitated making roadcuts into inherently unstable slopes. Mass wasting would be a fact of life here, but a major paved highway would only make the situation worse. The highway has been shut down for months at a time while landslides were mitigated.

I am guilty sometimes of ignoring interpretive signs on the assumption that I supposedly know more than the sign-makers (a poor assumption, actually). The picture below (from a sign at the overlook) shows McWay Falls as it appeared prior to 1983. The cove was full of seawater, not sand! What happened?

In 1983, a huge mass wasting event shut down Highway 1 just north of the park for nearly a year, and repair crews dumped 8 million cubic yards of debris down the slopes below to stabilize the highway. Wave action immediately began carrying the debris south along the coast, forming sandy beaches in the protected coves where there had been no sand before. The slope is still contributing massive amounts of sand to the coastal system (the yellowish debris on the right in the picture below).

In a few years or decades, the loose debris will have been carried off and wave action will start removing sand from the coves. Some geological changes happen on human timescales.

Julia Pfeiffer Burns State Park is on the Big Sur Coastline about 37 miles south of Carmel on Highway 1. Walk-in campsites are available, and a major campground can be found 12 miles north at Pfeiffer Big Sur State Park (we had a pleasant stay there during our visit this week). The park has several trails to explore beyond the overlook.

Accretionary Wedge #10: Geology and Art

I almost missed the month's version of the Accretionary Wedge, hosted by John at Geological Musings in the Taconic Mountains! I am leaving for the Colorado Plateau VERY early in the morning, and will have very limited web access for the next two weeks. I feel like I will be missing a lot in the geoblogosphere, and fully expect to find a dozen or more geology blogs starting up as students around the country become inspired by their time in the field. I will be anxious to see what comes up in the next few weeks!

The Accretionary Wedge topic for the month, Geology and Art is a great one, and one that deserves a great deal more thought, but with the crush of packing, I can only offer a couple of thoughts off the top of my head...

The best geology-related song: Landslide by Stevie Nicks and the Dixie Chicks (and probably others). I love the song on the merits, but it is nice to see a geological process used as a metaphor for drastic life changes.

"I took my love, I took it down

Climbed a mountain and I turned around

I saw my reflection in the snow covered hills

till the landslide brought me down....

Well, I've been afraid of changing

'cause I've built my life around you

But time makes you bolder

Children get older

I'm getting older too"

I guess I must be a little sentimental these weeks, having reached my fifties, and watching my son walk down the aisle with his wonderful new wife, and watching my daughter achieve success in her field, even if it isn't geology ;).

The Best in Painted Media: The Mona Lisa by Leonardo Da Vinci

My wife, the art teacher, tells me that Da Vinci was one of the earliest painters who made effective use of natural settings in his art. So much is going on in the painting of the Mona Lisa, including the rocky peaks and meandering rivers. I am not all that well-versed in art, although in my travels with my wife, I have certainly developed an appreciation for art history, just as she has developed an appreciation for geology. For that matter, so did Da Vinci.

We had the privilege and opportunity a few years back to see the Louvre, and because we arrived very early in the day, we were able to linger in front of the Mona Lisa for much longer than would be normal for such an iconic image. It was a moving experience, one of many in the three days we had in Paris.

It has nothing to do with anything else, but did you know there is an entire 12th-century medieval castle buried underneath one of the wings of the Louvre, and that it has been excavated to the extent that you can walk around and through it? I had no idea when I walked into the museum that I would be exploring a castle too!